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O. A. COLBY.

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'1 ,306,25 1 mmm .rune 10, 1919.

INVENTOR 'am A. (C7/; BY

ATr'oRNEY UNITED STATES PATENT OFFICE.

ORA A. COLBY, 0F LARIMER, PENNSYLVANIA, ASSIGNOR TO WESTINGHOUSE ELECTRIC AND MANUFACTURING COMPANY, A CORIORATION OF PENNSYLVANIA.

ELECTRIC FURNACE.

Specification, of Letters Patent.

latented'Junc 10, 1919.

Application tiled October 24, 1917. Serial No. 198,281.

To au Iwhom t may concern:

Be it known that I, ORA A. CoLBY, a citizen of the United States, and a resident of Larimer, in the county of Westmoreland and State of Pennsylvania, have invented a new and useful Improvement in Electric Furnaces, of which the following is a speciication.

This invention relates to electric furnaces, and, in particular, to an electric-resistance, high-temperature furnace for heat-ing and treating metals and also for other. heating applications, such as tiring ceramics and enameled ware. The furnace construction herein disclosed is particularly adapted for heating steel to forging temperatures.

I have found that the impurities in the granular resistor bed of an electric-resistance furnace are gradually oxidized and consumed at the high temperatures of the furnace, especially in the portion of the bed forming the floor of the heating chamber or hearth. This results in a rapid increase in resistance and consequent failure to heat, and it is an object of this invention to provide an improved resistance-furnace construction that shall eliminate this undesir-V vline II-II of Fig. 1; Fig. 3 is a sectional view of a portion of another electrieresistance furnace embodying this invention; and Fig. 4 is a sectio "l view of the furnace shown in Fig. 3, t ken on the line IV-IV of Fig. 3.

Referring to Figs. 1 and 2. the furnace construction comprises an exterior iron frame 10 which incloses the fire-brick furnace walls 11 and the 'fire-brick floor 12 -which supports the resistor of the furnace.

The heating chamber or hearth 13 has an arched roof 14 which is carried by walls 15 having parts thereof spaced from the walls 11 to form the hoppers 16. for a purpose to be described.

The resistor of the furnace constitutes the floor of the heating chamber 13 and comprises a layer 17 of non-"fusible electrical conlayer 17 of graphite is superposed on a.

layer 170 of charcoal or similar material that is inclosed, at the bottom and sides, by a lining 19 of refractory material, such as a composition of carborundum sand and silicate of soda, or carborundum sand, graphite and Crucible clay, forming a container of refractory heat-insulating material supported on the lire-brick base 12. Since carbon has a negative temperature coefficient, the current through the resistor will tend to concentrate in the zone of highest temperature, which I have found to be near the central port-ion of the resistor because that part is completely inclosed. The portion of the resistor which is near to, and exposed in. the heating chamber 13 will be cooler and, if it is substantially of the samematerial as the lower portion of the resistor, it will have a greateiresistance to the flow of current. and the resist-or will not heat up uniformly. For example. I have found that, where the resistor consists entirely of granular material. the temperature of the interior portion is about 20000 C. when a temperature of about 1000o (l. is maintained on the surface of the hearth in the heating chamber. Such a high temperature in the interior exceeds the safe limit of the refractory material 19 and` in order to avoid such a wide difference of temperature. I found it desirable to use a material in the upper portion of the resistor of less resistance than the' granular material so that the resistance and temperature between the ends of the, resistor and the eleru trodes are filled with powdered or granular carhonaceous material, such as graphite,

which is packed around the electrodes 20 to maintain a good contact therewith and to prevent overheating or arcing. The masses of granular non-fusible material between the ends of the resistor and the electrodes are of considerably greater cross-section than the resistor bed and will thus form thermal-insulating and electrical-conducting bodies between thc resistor and the electrodes. This granular material is oxidizable and will be gradually consumed, especially that portion near the resistor, but the same may be replaced by new material vcharged into the upper ends of the hoppers 16. The granular material is consumed very slowly and, in a furnace designed according to this invention, as I have found that the rate of consumption was only about two pounds per day, the hoppers may be filled to the top and the furnace operated for a long period of time With- Vout much attention other than an occasional tampingof the granular material to pack the conducting masses between the resistor and the electrodes. This form of furnace construction is therefore adapted to be operated continuously and economically for a comparatively long period of time without shutting down the same for cleaning or repairs. The expense of operation is also much reduced because the expensive electrodes 20 are completely inclosed or sealed from the atmosphere by the granular masses 22. Y The ca'rborundum bricks 18, comprising the floor of the heating chamber, have a large negative temperattire-resistance coefcient, are highly refractory and deteriorate very slowly. A

In Figs. 3 and 4, I have illustrated a modified ,form of construction embodying the principles of my invention and, in most respects, similar to the furnace construction illustrated in Figs. 1 and 2. The principal difference is that, instead of the transversely extendingrectangular carborundum bricks 18, I employ a series of bricks 181 of wedgeshape and composed of graphite, amorphous carbon or carborundum embedded in, and

extending lengthwise of, the resistor bed 171 and arranged with their tapering "ends eX- tending downwardly. The wedges 181 are 'placed in a' series, side by side, slightly below the upp'er surface of the resistor bed 171 and, since the `narrowest portions .thereof arelowest, the resistance of the resistor will gradually decrease from the bottom ofthe bed 171, rendering the resistor substantially of uniform resistance. Y

I have discovered that silicon carbid or carborundum, although having a larger negative' temperature-resistance coefficient than carbon, is, nevertheless, very suitable for use as a resistor, since it resists oxidation up to an extremely high temperature, is rigid and of good heat conductivity and is available in various molded shapes. Solid blocks of graphite or carbon have better thermal conductivity than carborundum but are objectionable because of the combustion which takes place on the exposed surfaces, and the difficulty of repair or replacement of burned portions thereof.

While I have shown and described the construction of the furnace in detail, it is to be understood that various changes may be made therein without departing from the spirit of the invention and the scope of the appended claims.

I claim as my invention:

1. In an electric furnace, the combination with a heating chamber, of a resistor adjacent to said chamber and comprising refractory electrical-conducting block material, terminal electrodes, and a mass of thermal-insulating and electrical-conducting material interposed between .each of said electrodes and the resistor for conducting current therebetween and adapted to prevent overheating and consumption of said electrodes.

2. In an electric furnace, the combination with a heating chamber, of a resistor below said chamber comprising a plurality of blocks of electrical-conducting and refractory material, terminal electrodes, and a mass of thermal-insulating and electricalconducting granular refractory material interposed between each of said electrodes and the resistor and adapted to prevent overheating and consumption of said electrodes.

3. In an electric furnace, the combination with a heating chamber, of a resistorbed forming the floor of said chamber and comprising a body of granular material having solid conducting block material embedded therein, termina-l electrodes, and a mass of V cal-conducting granular material interposed between said electrodes' andthe ends of said resistor bed, said masses of material' preventing the overheating of said electrodes.

In an electric furnace, .the combination with a heating chamber, of a resistor forming the oor of said chamber and comprising a bed of thermal-insulating and electricalconducting granular carbonace'ous material having a series of blocks of carbonaceous material embedded in' the upper portion thereof to render `said portion of less resistance than the lower portion of thebed,

terminal electrodes of solid carbonaceous material having the ends thereof disposed adjacent to the ends of said resistor, and a mass of thermal-insulating and electricalconducting carbonaceous material interposed between each of said electrodes and the resister, said mass being granular and preventing overheating and consumption of said electrodes.

(3. In an electric furnace, the combination with a heating chamber, 'of a resistor comprisinga bed of granular material having a series of blocks of refractory electricalconducting material embedded in a prtion thereof adjacent said chamber, terminal electrodes, a mass of thermal-insulating and electrical-conducting granular material interposed between said electrodes and the ends of said resistor to prevent overheating of the electrodes, and means whereby material may be added to said mass to replace that which is consumed.

7. ln an electric furnace, the combination with a heating chamber, of a resistor comprising a bed of granular material having a series of blocks of refractory electrical-conducting material embedded in a portion thereof adjacent to said chamwer, terminal electrodes, a mass of thermal-insulating and electrical-conducting granular material interposed between said electrodes and the ends of said resistorto prevent overheating yof the electrodes, and hoppers for containing granular material to be fed to said mass to replace that which is consumed.

3. ln an electric furnace, the combination with a heating chamber, of :a resistor comprising a bed of granular material having a series of blocks of refractory electricalconducting material embedded in the portion thereof adjacent said chamber, hoppers adjacent the opposite ends of said resistor, tern'iii'ial electrodes in said hoppers and a mass of thcrmail-insulating and electricalconducting granulaiinaterial interposed between said electrodes and the `ends of said resistor to prevent overheating of said electrodes', said hoppers having inlets whereby granular material may be charged into 'the same to replace the portion of said material that is consumed.

f). 1n an electric furnace, the combination with a heating chamber, of a resistor constituting the floor of the chamber and comprising a bed consisting of a layer-of carbonaceous material, a layer of graphite above the layer of carbonaceous material, and a series of carbonaceous blocks embedded in the graphite layer, a lining of heat-insulating and eleetrical-conducting material surrounding the bottom and sides of said hed. hoppers adjacent the opposite ends of said resistor, carbonaceous terminal electrodes in said hoppers, and a mass of thermal insulating and electrical conducting granular'material interposed between said electrodes and the ends of said resistor and of greater cross-section than the latter to prevent overheating of said electrodes, said hoppers having inlets whereby granular inaterial may be charged into the same to replace that portion of said material that is consumed.

10.' An electric-resistance furnace coinprising a resistor consisting of a granular bed of earbonaceous materialv and solid carbonaceous block material embedded in said granular material.

11. An electric-resistance furnace having a heating chamber provided with a wall comprising blocks of carbid composition exposed in said chamber, said wall constituting the resistor of the furnace.

12. An electric-resistance furnace having a heating chamber provided with a floor comprising blocks of silicon carbid exposed in said chamber, said floor constituting the resistor of the furnace.

13. An electric-resistance furnace having a heating chamber provided with a Wall comprising blocks containing material that has a larger 'negative temperature-resistance coefficient and oxidizes less rapidly than carbon, said Wall constituting the resistor of the furnace.

14. An electric-resistance heating device comprising a resistor consisting of blocks composed of material that has a larger negative temperature-resistance coefiicient than carbon and oxidizes less rapidly than carbon.

15. An electric furnace comprising a heating chamber, one of the walls of which consists of refractory electric-alconducting block material to constitute a resistor, terminal electrodesand a mass of thermal-insulating electrical-conducting material interposed between each of said electrodes and the ends of the resistor to prevent overheating of the'electrodes.

16. An electric furnace comprising a heating chan'iber, the floor of which consists of blocks of refractory electrical-conducting material in contact with one another to constitute a resistor, terminal electrodes, and a mass of thermal-insulating and electricalconducting material interposed between each of the electrodes and the ends of the resistor, said mass being granular and of greater cross-section than the resistor to prevent the overheating and consumption of the electrodes.

ln testimony whereof, I have hereunto subscribed my name this 18th day of Oct., 1917.

ORA A. COLBY. 

